The invention relates to an aqueous composition usable as a hydraulic medium in hydrostatic transmission of power. The invention also relates to the use of certain salts and their aqueous solutions as a hydraulic medium and in a hydraulic medium.
Hydraulic media used for transmitting energy in hydrostatic power transmission play a substantial role in hydraulics. In the first hydraulic applications taken into use, water was used as the medium. Water has certain commonly known disadvantages, such as freezing at low temperatures, poor lubrication properties, and it also forms a favorable growth environment for microbes, from which there results formation of precipitates, bad odor, corrosion, and even generation of hydrogen, which may cause hydrogen brittleness in structural materials.
These disadvantages have been eliminated in hydraulics currently in use by using as the medium, for example, mineral oils, and recently also vegetable oils, to decrease the harm caused to the natural environment by mineral oils. Both vegetable and mineral oils have the disadvantage that their viscosities increase to detrimentally high levels at low temperatures. For most of the hydraulic oils, the lowest possible operating temperature is between xe2x88x9220 and xe2x88x9230xc2x0 C. Oils and fats have a further disadvantage in that they form a growth medium for microbes. Combustibility is also a serious common detrimental factor because of which efforts are being made to reduce the use of oils.
U.S. Pat. No. 5,451,334 discloses a possibility to avoid the untoward properties of mineral oils by using as the main component purified rapeseed oil or soybean oil, to which an anti-oxidant in an amount of 0.5-5% and large-molecular esters in an amount of at minimum 20% have been added. In this manner the congealing point of the oil can be lowered from the normal xe2x88x9216xc2x0 C. to as low as xe2x88x9240-xe2x88x9245xc2x0 C.
WO-9726311 discloses a heavy fluid, intended mainly for oil drilling, the fluid consisting of a polymer to increase the viscosity of the aqueous solution and an aqueous solution containing cations and anions so that the solution has either two cations or two anions. The objective is a fluid having high stability with respect to aging, heat, mechanical stress and slide stress.
DE-19 510 012 discloses a salt solution or a fluid medium for use in fluid circulations such as solar energy units, heat pumps, thermostats, vehicle cooling circulations or hydraulic circulations intended for heat transmission, or in pipes in which hot or cold energy can be transmitted in a fluid medium and there is the risk of the temperature dropping below the freezing point of water or the congealing point of the medium. The salt solution or the fluid medium is a mixture of water and a salt of propionic acid. According to the publication, in the manner disclosed the disadvantages of mixtures of water and glycol and mixtures of water and chloride, which have been used previously, are avoided and the foodstuff provisions are complied with. The medium disclosed does not burn or explode and is odorless, and according to the publication it can be used at temperatures as low as xe2x88x9250xc2x0 C. One further use suggested is aqueous hydraulics.
It has now been observed, surprisingly, that most of the disadvantages of prior art in hydraulics can be avoided by using as the hydraulic medium formate solutions which contain salts of formic acids. Such solutions are already used at present, for example, as heat transmission media owing to their environment-friendliness, low freezing points and also low viscosities at low temperatures.
According to the invention, there is thus provided an aqueous composition for use as a hydraulic medium in hydrostatic power transmission, which composition comprises an aqueous solution of a salt of formic acid.
The salt of formic acid may be an alkali metal salt of formic acid such as a sodium or potassium salt, an alkaline-earth metal salt such as a calcium or magnesium salt, or an ammonium salt. Potassium formate is an especially advantageous salt of formic acid.
The suitable concentration of the salt of formic acid is approx. 1-75% by weight, preferably approx. 5-60% by weight, and especially preferably approx. 10-60% by weight, depending on the targeted use.
The aqueous composition according to the invention may additionally contain additives, such as a thickener and/or a corrosion inhibitor.
The suitable amount of thickener is approx. 0.1-5% by weight, preferably approx. 0.2-1.0% by weight. Preferable thickeners include acrylic acid polymers and co-polymers, of which sodium polyacrylate and a copolymer of sodium acrylate and acrylamide can be mentioned.
The invention also relates to the use of an aqueous solution of a salt of formic acid as a hydraulic medium.
The invention additionally relates to the use of a salt of formic acid in an aqueous hydraulic medium, wherein the salt of formic acid has an inhibiting effect on microbial growth and a lowering effect on the freezing point, as well as a lowering effect on viscosity at low temperatures.
In comparison with the mixture of water and a salt of propionic acid proposed in DE published application 19 510 012, the following advantages are achieved with the aqueous solutions of salts of formic acid according to the present invention, i.e. formate solutions.
Formate solutions have lower freezing points. It is stated that with the sodium propionate solution disclosed in DE-19 510 012 a temperature of xe2x88x9250xc2x0 C. can be attained, whereas with a 50-percent potassium formate solution a temperature of xe2x88x9260xc2x0 C. can be attained, and temperatures even lower than this can be attained by using a stronger solution.
Furthermore, the viscosities of formate solutions at low temperatures are lower than those of corresponding propionate solutions. For example, the viscosity of a 50-percent aqueous solution of potassium formate at xe2x88x9240xc2x0 C. is 18.9 cSt.
Formic acid, which is used for the preparation of formates, is more effective against microbes than is propionic acid. For example, against the Pseudomonas putila bacterium the EC 50 value of formic acid is 46.7 mg/l and the corresponding value of propionic acid is 59.6 mg/l. It can be assumed that the ratios of the corresponding property of corresponding salts of the said acids are the same.
The invention is described below in greater detail with the help of examples. The percentages given in the present specification are percentages by weight, unless otherwise indicated.